1. Field of the Invention
The present invention relates to a magnetizing apparatus for use in a thermoremanent magnetographic printer and more particularly to a premagnetizing station in such a printer having energy efficient magnetizing apparatus therein that produces magnetization in the magnetic recording medium of the printer that is substantially devoid of a magnetization component normal to the plane of the medium.
2. Description of the Prior Art
It is known that heating a ferromagnetic material above a certain temperature will cause it to become paramagnetic. The temperature at which the ferromagnetic material loses its ferromagnetic properties is referred to as the Curie temperature. Normally, reversing the process or cooling the material below the Curie temperature will restore the ferromagnetic properties.
One important parameter of a ferromagnetic material affected by the temperature-induced phase change is a loss of remanent magnetization stored in the ferromagnetic material before it is heated. Generally, after the application of a sufficiently large magnetic field to a ferromagnetic material and its removal, the material will show a magnetic field of a certain magnitude and polarity that is remanent or remains. However, when a material having a remanent magnetization is carried into the paramagnetic phase by heating, the remanent magnetization is lost. Thus, the heating of a ferromagnetic material beyond its Curie temperature is used for erasing magnetization stored in a material. Further, since the coercivity of a ferromagnetic material is also a function of temperature and decreases to zero at the Curie point, the heating of a ferromagnetic material beyond its Curie point and cooling it in the presence of a magnetic field is a method of recording a magnetization therein based upon the applied magnetic field.
The formation of a magnetic latent graphic image on a magnetizable surface by thermomagnetic recording is well known in the art. U.S. Pat. No. 3,522,090 and U.S. Pat. No. 3,554,798, both to G. R. Nacci, relate to magnetic recording members particularly adapted for use in thermomagnetic recording and copying. U.S. Pat. Nos. 3,555,556 and 3,555,557, both also issued to R. Nacci, are illustrative of references that describe the process and apparatus for reflex recording of optical images on magnetic media. U.S. Pat. No. 3,698,005 to G. R. Nacci et al describes a recording member for reflexive imaging where a magnetic material is heated beyond its Curie temperature by a flash exposure procedure.
Thermoremanent magnetic imaging, from the above discussion, is simply an imaging technique that creates a latent magnetic image on a ferromagnetic material that is usually, but not necessarily, coated on an insulating substrate. The image is created by locally heating portions of the coating material above the Curie temperatue point to achieve a phase change in magnetic properties and simultaneously applying a magnetic field so that as the coating material cools in the presence of the applied magnetic field, the remanent magnetization from the applied field remains in the coating material, resulting in a latent magnetic image in the coating material. Such a latent magnetic image may be developed with magnetic toner, the toner transferred being in image configuration to a recording sheet such as paper and fixed thereto for a permanent copy.
U.S. Pat. No. 4,294,901 to Genovese discloses a multi-layered substrate for thermoremanent magnetic imaging. A conductive stylus array provides current between selected styli and through the substrate to heat locally selected portions in image configuration to the Curie temperature of the substrate. A magnetic latent image is formed when the heated portion of the member is allowed to cool in an externally applied magnetic field at a strength of between 10 and 200 gauss. In another embodiment, the substrate is premagnetized and the background image areas of the substrate are heated to the Curie temperature. The substrate is thereafter cooled in the absence of any externally applied magnetic field. In each embodiment, the latent image is developed by contacting the substrate containing the latent image with magnetic toner and transferring the developed image to a permanent sheet of, for example, paper and fixing the developed image thereto.
U.S. Pat. No. 3,804,511 to Rait et al. teaches the use of a tape having a recording medium on the surface which is magnetizable and capable of forming an electrostatic image. After a latent electrostatic image is formed and the image on the recording medium developed with toner having both an electrostatically attractive component and a magnetic component, the side of the tape opposite to the one containing the developed image is subjected to a continuous AC magnetizing current which applies a uniform magnetic recording in the recording medium through the tape. A latent magnetic image is formed in the recording medium by applying a magnetic erasing signal to the tape from the side of the tape confronting the developing image through the toner image, so that the toner shields the recording medium and only the non-image area of the recording medium is erased. Therefore, a latent magnetic image corresponding to or duplicating the latent electrostatic image is formed. The aim of Rait et al. was to enable multiple copies to be made from one latent image. After the first image was produced electrophotographically, the second and subsequent copies were obtained by developing the latent magnetic image with the same toner (that has a magnetic component) and transferring the developed image to paper without removing the latent magnetic image which is retained in the recording medium until specifically erased by another magnetic erasing signal applied after the last developed image is transferred to paper.
U.S. Pat. No. 4,032,923 to Pond et al. discloses a thermoremanent magnetographic imaging apparatus which copies xerographically produced images from a slave web onto a magnetizable surface of a master web. The termomagnetic transfer is produced by exposing the slave and master webs while in intimate contact to a single intense burst of radiation from a Xenon lamp. The master web is pre-recorded in alternating patterns of magnetization by an AC recording head. The frequency at which the record head is gated by the alternating current source determines the final image resolution. The radiant energy from the lamp raises the temperature of the master web above its Curie point in the non-image areas, thus erasing the pre-magnetization pattern of alternating magnetic pole directions in the non-image areas. The remaining image is then developed by magnetic toner and transferred to a copy sheet.
U.S. Pat. No. 4,343,008 to Swigert discloses a method of making a magnetic imaging master capable of development with magnetic toner and transfer of the developed image many times. The master is made by pre-magnetizing the master and inserting it into a conventional typewriter where character images are typed on a backing layer of the master creating a right reading image therein. The master is then flash exposed to a Xenon lamp which erases all pre-magnetized areas not shielded by the typed characters.
U.S. Pat. No. 3,791,843 and U.S. Pat. No. 3,845,306 to Kohlmannsperger discloses method and apparatus, respectively, for thermomagnetic imaging. These cases disclose the use of particular compounds such as Fe Rh, as a coating on the magnetic recording medium. Such compounds are antiferromagnetic at temperatures above and below a particular temperature known as the Neel temperature. In one embodiment, the coated recording medium is on a rotatable drum which is internally heated to its Neel temperature, and an original document is radiantly exposed on the recording medium at an exposure station in the typical successive incremental fashion well known in the art. The radiant exposure source emits radiant energy which passes through the original document, impinging on the recording medium. Those portions of the recording medium which register with the image-free portion of the original document are heated by the radiant source to a temperature above the normal Neel temperature. Consequently, such portions exhibit greatly reduced coercive force which is so weak that magnetic toner will not be retained at a development station.
Copending U.S. application entitled "Thermoremanent Magnetic Imaging Method" by R. E. Drews et al filed July 20, 1983 as Ser. No. 515,720 and assigned to the same assignee as this application discloses a thermoremanent magnetic imaging method and apparatus, wherein, among other features, the recording medium is pre-magnetized prior to entry of the medium into the printing or imaging station. Thus, it is known from the above that pre-magnetization of a magnetic recording medium can be accomplished with either a permanent magnet, an electromagnet, or an alternating recording head. The magnets produce a magnetization in the magnetic recording medium having magnetic pole directions all oriented in the same direction, while the alternating recording head or alternating electromagnets produce alternating rows of opposite magnetic pole directions. Since latent magnetic images are produced by magnetic fringe fields, the pre-magnetized recording medium must contain adjacent small regions of magnetization having opposite pole directions. The AC pre-magnetization is simply erased in the background regions by, for example, heating the recording medium above its Curie temperature. When the pre-magnetization has its magnetic pole direction all in the same direction, the imaging method taught by abovementioned application to R. E. Drews must be used, so that the recording medium is heated in image configuration to erase the pre-magnetization and subequently cooled in the presence of a magnetic field of opposite polarity to that of the pre-magnetization. The heating, of course, is done by one row of pixels at a time with a space therebetween. In this manner, small regions or pixels having opposite magnetic pole directions are formed.
The disadvantage of the single horeshoe permanent magnet, as disclosed in the prior art, is that the fringe field it produces always has a sizable component normal to the plane of the tape. This is undesirable because it weakens the development fields subsequently produced. The recording medium may also be pre-magnetized, according to the prior art, by passing it through a large DC powered solenoid, but such a solenoid requires around 30 watts to sustain a 500 oersted field across a 10 inch wide recording medium. Another drawback with the solenoid approach is that it is awkward to use with a pre-fabricated endless recording medium because it must pass through the solenoid.
The prior art discloses, alternatively, that the reocrding medium can be pre-magnetized by employing a pair of opposing, recording head-like electromagnets. However, to generate the required field over a 10 inch wide recording medium with a reasonable gap or spacing therebetween of about 0.1 inch requires around 20 watts during the pre-magnetization operation. In all known prior art, either there is a significant power requirement or there is an undesirable magnetization component normal to the plane of the recording medium present during the pre-magnetization process.